Laser frequency doubling


Laser frequency doubling describes the laser whose wavelength is decreased by half, as well as the frequency is doubled by the frequency doubling crystal (LBO, BBO). After the crystal doubles the frequency of 1064nm solid light, it comes to 532 green light.

Doubling condition

The condition for frequency doubling is that the crystal can get instructions to make sure that the basic frequency laser with frequency f1 as well as the frequency doubled light with frequency 2 * f1 can have the very same refractive index (photon energy conservation), to ensure that excellent gain quality can exist in the crystal size. The laser can continuously transform the power from the f1 essential frequency to the 2 * f1 frequency doubled light.

The principle of optical frequency doubling

The conceptual basis for the frequency doubling of light is the nonlinear impact of laser light. The laser light is so intense that it causes the atomic polarization of the crystalline product, that is, the splitting up of favorable and unfavorable fee centers. This splitting up is a vibrant resonance, and the vibration frequency is consistent with the frequency of the laser. The resonance amplitude is connected to the intensity of the laser field. Since the laser magnetic field intensity and polarization strength are nonlinear, for second-order nonlinearity, the polarization intensity is symmetrical to the square of the laser’s electrical field intensity E.

The intensity of the essential frequency optical field varies, which can be seen from the trigonometric feature, Cosa * cosa= 0.5 *( cos2a +1). The second-order nonlinearity will produce double-frequency polarized resonance as well as zero-frequency polarized bias. This frequency-doubled polarization (resonance of the range in between positive and unfavorable charges) will create frequency-doubled light or contribute to obtaining the passing frequency-doubled laser light.

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Frequency-doubled light problem.

This improvement or enhancement of doubled-frequency light needs to meet 2 problems:

(1) The basic frequency light is ahead of the doubled frequency light by 0.75 π;

(2) The phase difference space remains unchanged in the crystal activity area.

The stage difference area remains the exact same, calling for the product to have the same refractive index for both frequencies. Generally, the refractive index of materials enhances with light frequency.

BBO crystals like this can meet the exact same refractive index in a specific direction. The constant refractive index ensures that the spatial combining area with a particular length in a certain direction in the crystal is dealt with and also the waveform distinction is steady. There is a specific variance in practice, so the combining length is limited, which is the particular size of the laser crystal.

Category of frequency-doubling crystals.

Ammonium dihydrogen phosphate (ADP), potassium dihydrogen phosphate (KDP), potassium dihydrogen phosphate (DKDP), dihydrogen arsenate crucible (DCDA), and also various other crystals.

They are a depictive type of crystals that produce dual-frequency and other nonlinear optical results, appropriate for use in the near-ultraviolet-visible and near-infrared areas, and have a big damage threshold.

Lithium niobate (LN), salt barium niobate, potassium niobate, α-type lithium iodate, and various other crystals.

The second nonlinear electrical polarization coefficient is huge, as well as the refractive index of crystals such as LN and BNN is sensitive to temperature level, which is different from the temperature level change qualities of the diffusion impact. People can adjust the temperature level properly to attain non-critical matching.

Appropriate for the visible light region and mid-infrared region (0.4 μ-5μ). LN is prone to refractive index adjustment as well as photodamage under light; the damage threshold of BNN is higher than that of LN, however, the solid service area is larger, and the make-up is simple to transform, leading to poor optical harmony, and also huge crystals with excellent efficiency are hard to acquire.

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Potassium niobate has no solid solution In the melting area, it is feasible to acquire large crystals with uniform optical homes. α lithium iodate is an aqueous solution growth crystal, which can expand big crystals with excellent optical top quality, and the damage limit is greater than that of BNN crystals. The downside is that it has no non-critical matching capability.

Semiconductor crystals.

Semiconductor crystals consist of gallium arsenide, gallium arsenide, zinc sulfide, cadmium zinc oxide, selenium, etc. Their quadratic nonlinear electric polarization coefficients are higher than those of the initial two crystals as well as appropriate for bigger infrared bands.

Nonetheless, with the exception of selenium and tellurium, many crystals have no dual refraction effect and also can not accomplish placement matching.

BBO laser frequency doubling crystal

Borate, barium metaborate (β-BBO), lithium triborate (LiB3O5), etc.

Amongst them, Scientists efficiently created barium metaborate and lithium triborate crystals for the first time in the 1980s. And had the superior advantages of big nonlinear optical coefficients as well as high laser damages limit. It is an exceptional crystal material for laser frequency conversion, which has caused fantastic repercussions worldwide.

Ideal for ultraviolet wavelengths, consisting of KBF, etc, and also for deep ultraviolet wavelengths. The standard requirements for the sum frequency, distinction frequency, and optical specification oscillation impacts of nonlinear optical crystals are the same as those of dual-frequency crystals.

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